Numerous neurological illnesses produce symmetrical disturbances of gait that are characterized by stooped posture, increased time with both feet on the floor, and decreased velocity, cadence, stride, foot-floor clearance, and arm swing. Locomotion becomes hesitant and defensive. Many patients exhibit particular difficulty with initiation of gait, such that their feet appear to stick to the floor when they stand and attempt to walk (start hesitation). Only a few studies of gait initiation have been performed in normal people, and the neurophysiological mechanisms of start hesitation have been the subject of only a few reports. Start hesitation is particularly common in patients with Parkinson disease and diseases that affect the frontal lobes (e.g., Binswanger disease and normal pressure hydrocephalus), but may be seen in other conditions (e.g., Alzheimer disease). Multiple mechanisms of start hesitation are therefore assured, but there are few clinical tools for deciphering these mechanisms in a particular patient. Rapid gait initiation consists of a sterotyped sequence of postural shifts that propel the body into forward motion. These postural shifts are produced by patterns of muscle activity in the lower extremities that resemble the patterns associated with voluntary and involuntary postural sway about the ankles. Impaired cognition, insufficient postural control, inability to perform skilled sequential movements, and impaired integration of postural control and movement are all possible mechanisms of start hesitation that we propose to explore in a series of neurophysiological experiments in neurologically healthy young and older people, patients with Alzheimer,disease, and patients with Parkinson disease. These experiments will use choice reaction time paradigms, computer-controlled infra-red stroboscopic cameras, floor-mounted force plates, and electromyography to examine the kinematics and electromyographic patterns of 1) gait initiation, 2) involuntary and voluntary postural sway, 3) anticipatory postural activity associated with voluntary movement of the upper extremities, and 4) the execution of stepping movements while sitting in a chair. Since many patients with start hesitation exhibit a combination of dementia and parkinsonian features, the data from our Alzheimer and Parkinson patients will be used as standards for comparison with data from elderly patients with other neurological conditions. This work will be useful in the design of rehabilitative programs and in deciphering the pathophysiology of impaired locomotion in older people.